1. Field of the Invention
The present invention relates generally to integrated circuits and, more particularly, to methods and apparatus for polishing wafers in chemical mechanical polishing systems.
2. Description of the Related Art
In the fabrication of semiconductor devices, wafers are typically processed through a number of well known process operations. Some of the conventional process operations include oxide deposition operations, metallization sputtering operations, photolithography operations, etching operations and various types of planarization operations. Because a semiconductor device is fabricated as a multi-level device that may have a number of metallization levels (and oxide levels in between), the need to planarize some of the layers before a next layer is applied becomes very apparent when the topographical variations start to increase. Consequently, if the topographical variations become too pronounced, the fabrication of additional levels may become restrictive, in that the topographical variations can limit the degree of precision needed to fabricate dimension sensitive integrated circuit devices.
One common planarization technique is referred to as chemical mechanical polishing (CMP). FIG. 1A shows a simplified drawing of a CMP apparatus 100 that functions as a polishing system is used to planarize various material layers that may be applied to a wafer 102 during a fabrication process. As is well known, the CMP apparatus 100 includes a robot arm 108 that has a wafer carrier 106 for handling the wafer 102 during a polishing operation. As shown, the actual planarization of the wafer 102 occurs when the robot arm 108 that functions as a mechanical arm for lowering the wafer carrier 106 down to a polishing pad 104. To complete a planarization operation, the polishing pad 104 is usually conditioned (i.e., to maintain the polishing pad's texture) before each new planarization operation is performed and a polishing slurry having a specific PH level is applied to the surface of the polishing pad 104. Once the polishing pad 104 is rotating at a given rpm, the wafer carrier 106 is lowered and placed in contact with the polishing pad 104. Once contact is made with the polishing pad, the CMP apparatus 100 will supply a back pressure (BP) to a back surface 103 of the wafer 102.
FIG. 1B shows a more detailed view of the wafer carrier 106 of FIG. 1A. This detailed view shows that the back pressure (BP) is conventionally applied to the center region of the wafer 102. As a result, when the wafer 102 is compressed against the polishing pad 104 during a planarization operation, the center region of the wafer 102 will polish at a faster rate than the outer regions 102a. In addition, it has been observed that prior art wafer carriers 106 have a lip 105 that prevents the back pressure (BP) from being applied to the edges of the back surface 103 of wafer 102. As a result, even though the applied back pressure (BP) is constant in a cavity 107, the pressure (P) applied to the back surface of the wafer is not, and therefore, non-uniform polishing rates over the surface of the wafer 102 have become increasingly problematic.
As mentioned above, the non-uniform polishing rates are most evident in the topographical variations that remain at the edge of the wafer 102. This topographical variation is a particular problem in the fabrication of shallow trench isolation (STI) where this is about 4 mm edge effect with remaining oxide due to a polishing pad re-bound effect at the edge of the wafer 102. The down side to the topographical variations is that many of the dies at the edge of the wafer 102 will become damaged, and therefore, will be unusable. In fact, for larger size wafers, the number of damaged dies on a particular wafer 102 will increase, thereby driving up the fabrication costs and reducing throughput.
In view of the foregoing, what is needed is a chemical mechanical polishing system that is capable of programmably controlling the back pressure (BP) that is applied to the back side of a wafer during a chemical mechanical polishing operation.